This invention relates to dynamic optical wavelength power equalizers and, more particularly, to reducing thermal crosstalk.
It is known that changing the temperature of a planar lightguide circuit (PLC) can affect the index of refraction so that when the PLC is used, for example, as a Mach-Zehnder Interferometer (MZI), the change in refractive index can be used to adjust the phase delay. Likewise, changing the index of refraction can be used to vary the coupling ratio of a directional coupler used in Fourier filters and ring filters. To vary refractive index, a heater can be deposited on the surface of the PLC. Generally, a substantial temperature change is required to effect the desired adjustment.
In the above mentioned copending Cappuzzo 7 application, the speed of response to a temperature change is increased by forming the heater strip on the upper cladding of a dual clad planar waveguide and by providing a heat channel between the heater and the waveguide core so as to reduce the thermal inertia. To minimize losses and achieve a PLC having small size, the spacings between components is made as small as possible. Unfortunately, small size tends to produce significant mutual coupling and thermal crosstalk. It would be advantageous to be able to retain small size and close component spacing without introducing crosstalk.
In accordance with the principles of our invention, in one aspect thereof, an illustrative PLC phase shifter is fabricated on a chip which exhibits reduced thermal crosstalk by reducing the thermal resistance between the waveguide cores and the thermal xe2x80x9cgroundxe2x80x9d (e.g., a heat sink) compared to the thermal resistance between the waveguide cores. This is accomplished by removing some of the glass from the backside of the chip over a small area under the phase shifters, depositing metal on the backside, and soldering it to a copper block heat sink.